Conventionally, as a cutting insert (hereinafter referred to as “insert” in some cases) used for performing a face milling process, an insert whose both hexagonal shaped surfaces are usable has been proposed (for example, refer to International Publication No. 2007-037733A1).
However, according to the insert of the International Publication No. 2007/037733A1, a major cutting edge 16 and a minor cutting edge 17 that are located with a corner cutting edge 20 interposed therebetween differ from each other in configuration as shown in FIGS. 1A and 1E thereof. Consequently, three corners of the hexagonal shape are respectively usable only in the same rotation direction (for example, the forward rotation direction), and hence there is a need to separately prepare a reverse rotation insert for ensuring a proper use.
On the other hand, a tip has been proposed which is usable in both the forward and reverse rotations by making both sides of a corner part 4 have the same cutting edge structure (for example, refer to Japanese Patent Unexamined Publication No. Publication No. 9-216113).
However, according to the tip of the Publication No. 9-216113, a portion 10 of a cutting edge used mainly during a cutting process is inclined in a straight line shape as shown in FIG. 2 thereof. It is therefore difficult to ensure sufficient cutting edge strength in the vicinity of the corner part 4 subjected to a large cutting force during the cutting process, and consequently a fracture may occur there. This is also true for the insert of the International Publication No. 2007/037733A1.
Hence, there is a need for an insert having both low cutting resistance and excellent fracture resistance.